High frequency circuitry, for example, for use at frequencies of greater than or equal to 500 megahertz (MHz) requires special engineering considerations as the wavelength of the signal is often the same approximate size or smaller than the length scale of the system under consideration. High frequency planar circuits, such as copper clad laminates are often passive devices, such as filters, whose performance is governed by the precise wavelength of the signal in the circuit. As such, the dielectric constant of the insulating, dielectric material in a copper clad laminate should be precisely controlled. It can also be important that the dielectric material also exhibits a low dielectric loss.
Polytetrafluoroethylene (PTFE) compositions have been widely used as the dielectric material in copper clad laminates. Commercially, PTFE is manufactured by two different methods: suspension polymerization and emulsion polymerization. Suspension polymerization results in “granular” PTFE. The granular material is generally used as a molding compound, both with and without fillers. A large number of PTFE objects are molded from granular PTFE. Granular PTFE can also be used in the manufacture of skived films, where skived films can be made by molding a billet of granular PTFE and skiving the film on a lathe.
Emulsion polymerization results in a PTFE dispersion. This dispersion comprises small (<0.3 micrometer (μm) diameter) particles of PTFE suspended in water. The dispersion can be concentrated and stabilized with a surfactant and used as a coating. PTFE dispersion coated glass fabric is widely used as industrial belting, architectural fabric, and high frequency circuit substrate. Alternatively, the dispersion can be coagulated, resulting in, not surprisingly, a coagulated dispersion or a fine powder PTFE composition. The PTFE fine powder has the unique property of being processable by “paste extrusion.” As described in U.S. Pat. No. 2,685,707, the PTFE fine powder can be blended with a hydrocarbon liquid, lubricated, and then forced through a contracting die. The extrusion step “fibrillates” the PTFE particles. The lubricant can then be removed by evaporation or extraction. In the years since, a number of variations of the paste extrusion process have been practiced, including paste extruding or molding a preform and calendering the preform to form sheets, as described in U.S. Pat. Nos. 4,335,180 and 4,518,737. For example, the DuPont HS-10 process involves calendering in two directions to form biaxially oriented sheets. U.S. Pat. No. 3,953,566 discloses the additional stretching of the paste extruded PTFE to form a porous film or fiber with increased porosity and enhanced matrix tensile strength.
Both granular and dispersion (or coagulated dispersion) grades of PTFE can be true homopolymer compounds (containing only tetrafluoroethylene monomer) or “trace modified homopolymer” compounds (that contain less than 1 weight percent (wt %) of a co-monomer).
3M Corporation has developed a dielectric material comprising approximately 50 volume percent (vol %) PTFE and 50 vol % of a titanium dioxide (TiO2) powder that was marketed under the trade name of EPSILAM 10. In forming the copper clad laminate, PTFE and a TiO2 powder were first blended and lubricated with a paraffinic hydrocarbon solvent such as ISOPAR (commercially available from ExxonMobil). Sheets were then formed by paste extruding and calendering the blended material. The lubricant was then removed by drying and the sheets were laminated to a copper foil under high pressure above the melting temperature of the PTFE in a flatbed press. The copper clad laminates were then selectively etched and machined to form high frequency electronic circuitry with a dielectric constant of approximately 10. Several other manufacturers developed similar materials, such as Rogers Corporation's RT/DUROID® 6010 laminate and Keene Corporation's 810 laminate.
In later years, high dielectric constant laminates were made by impregnating woven and nonwoven glass fabrics with a slurry comprising PTFE and a ceramic powder, for example, by dip coating the slurry onto woven glass fabrics or by casting the slurry onto a glass fabric followed by lamination to a copper foil. An example of such a method includes, woven glass PTFE laminates that were made by impregnating a glass fabric with a PTFE dispersion to form a PTFE-impregnated glass fabric. The PTFE-impregnated glass fabric was then laid up, for example, with additional unreinforced skived PTFE plies, and laminated to copper foil in a flatbed press above the melting point of the PTFE. Alternatively, non-woven glass PTFE laminates were made by co-coagulating glass microfibers that have been dispersed in water with a PTFE dispersion, forming a sheet on a papermaking machine, and laminating the sheet to a copper foil. An example, of such a non-woven glass PTFE composite is Rogers RT/DUROID 5870 PTFE laminate. U.S. Pat. Nos. 4,335,180 and 4,518,737 disclose a process where the filler and a small amount of glass microfibers were co-coagulated with a PTFE dispersion and dried. The mixture was then paste extruded and calendered to form sheets. The sheets were laminated to copper foil in a flatbed press under pressure at a temperature above the melting temperature of the PTFE. In all of these cases, the PTFE was melted (also referred to herein as sintered) during the lamination step.
U.S. Pat. No. 4,996,097 discloses the manufacture of a thin, high capacitance laminate with a PTFE dielectric layer. The claimed 0.0001 to 0.005 inch (2.54 to 127 micrometer) thick dielectric layer was made by the process disclosed in U.S. Pat. No. 3,953,566, where a conductive material, such as copper, was laminated to one or both sides of the PTFE dielectric layer using a conventional lamination procedure of pressing at a pressure of 1,000 pounds per square inch (psi) (6,895 kilopascal (kPa)) and a temperature of 350 degrees Celsius (° C.), a temperature that is greater than the melting temperature of PTFE. An alternative to lamination above the melting temperature of the PTFE composition is also disclosed where a thermoset can be present to lower lamination temperature and improve adhesion of the dielectric layer to the conductive metal.
In recent years, additional applications for low loss, high dielectric constant materials have commercially arisen. The miniaturization of antennas for handheld consumer electronic devices is one such application, where high dielectric constant values will allow for a great degree of said miniaturization. U.S. Pat. Nos. 7,773,041 and 8,427,377 describe a dielectrically loaded loop antenna. U.S. Pat. No. 8,599,072 discloses a broadband antenna structure, in which the dielectric constant of the dielectric material of the cover reduces the required size of the conductive antenna element. In many of these applications, it is not necessarily desired that the high dielectric constant, low loss substrate be bonded directly to copper foil.
Higher dielectric constant materials can be achieved by using higher dielectric constant ceramic fillers, for example, having a dielectric constant in the gigahertz (GHz) range of 35-500, such as barium titanate. Many of these fillers, however, significantly increase the dielectric loss of the material and they can be difficult and not cost-effective to fabricate. The magnitude of the dielectric constant of a PTFE material can further be increased by increasing the amount of dielectric filler, but the amount of filler can be limited by the amount of filler in powder form that can be incorporated into the material. For example, depending on the particle size distribution, the surface area, and surface chemistry of the filler, loadings of greater than or equal to 50 vol % can exceed a maximum packing ratio of the powder in the material and adding additional powder can result in an increased amount of voids becoming entrained, and the material can become brittle.
Currently, commercial PTFE-titanium dioxide copper clad circuit laminates can exhibit dielectric constant values in the GHz frequency range of 10 to 12, where a maximum achievable dielectric constant is about 13.
There remains an unfulfilled need for low loss, high dielectric constant PTFE materials that exhibit higher dielectric constant values.